1. Field of the Invention
The present invention relates to a nonaqueous secondary battery comprising a negative electrode capable of intercalating/deintercalating lithium ions, a positive electrode having a lithium-containing metal oxide as a positive electrode active material, a nonaqueous electrolyte, and a separator for separating the positive electrode and negative electrode from each other.
2.Description of the Related Art
In recent years, with remarkable development of reduction of size and weight of an electronic appliance, a battery which is a power source therefor is very greatly demanded to implement reduction of size and weight. In order to satisfy such a demand, a nonaqueous secondary battery which is small in size and light in weight, has a large capacity and is capable of charging/discharging (which is represented by a lithium battery) has been put into practice. This nonaqueous secondary battery has been used in portable electronic/communication appliances such as a small-sized video camera, portable telephone, notebook-size personal computer, etc.
This kind of nonaqueous secondary battery uses a negative electrode active material of a carbon material capable of intercalating/deintercalating lithium ions, a positive electrode active material of a lithium-containing transitional metal oxide such as LiMn2O4, LiNiO2, LiMnO4, LiFeO2, etc., and an electrolyte with lithium salt dissolved as solute in an organic solvent. In the nonaqueous secondary battery, after having been assembled as a battery, lithium ions derived from the positive electrode active material by initial charging enter carbon particles, thereby permitting charging/discharging to be realized.
Meanwhile, in the nonaqueous secondary battery, in which the nonaqueous electrolyte is used, a separator for separating the positive electrode and negative electrode from each other has been adopted which is a microporous film of a polyolefin resin such as a polyethylene (PE) or polypropylene (PP) which exhibits low reaction with the organic solvent and is inexpensive.
However, the microporous film made of polyethylene or polypropylene only keeps the electrolyte in its vacant holes. Therefore, it was problematic in that it exhibits low reltainability of electrolyte. In addition, the low electrolyte-retaining nature increases a internal resistance. This presented a problem of deteriorating overcharge characteristic as well as the cycle characteristic.
In order to solve these problems, for the purpose of improving the electrolyte-retaining nature, the non-woven fabric of polypropylene (PP) or polyethylene terephthalate (PET) was adopted as the separator. However, the non-woven fabric of PP or PET presented a problem of deteriorating the cycle characteristic like the microporous film.
This time, it was proposed to adopt the polyvinylidene fluoride resin film having good electrolyte-retaining nature. Since the polyvinylidene fluoride resin film provides good electrolyte-retaining nature and can be kept in intimate contact with the electrode, the internal resistance in the battery is reduced and the battery characteristic can be improved.
Although the polyvinylidene fluoride resin film provides good electrolyte-retaining nature, it presented a problem of a great change in the size due to swelling. Specifically, as shown in FIG. 5, when a separator 60 of the polyvinylidene fluoride resin film retains the electrolyte, it swells in all the directions of width (x-direction in FIG. 5), length (y-direction in FIG. 5) and height (z-direction in FIG. 5).
Meanwhile, as shown in FIG. 6, such a nonaqueous secondary battery described above is manufactured in such a way that a laminate composed of a positive electrode 70 (which is formed by applying a positive electrode active material layer 72 on both surfaces of a core body 71), a negative electrode 71 (which is formed by applying a negative electrode active material layer 82 on both surfaces of a core body 81) and a separator 60 located therebetween is wound spirally to provide a group of electrodes which are housed in a battery case. When the separator 60 swells, it wrinkles so that gaps 61 and 62 are produced between the separator 60 and the positive electrode 70 or the negative electrode 80. This led to a problem of an increase in the internal resistance. When the separator 60 wrinkles so that gaps 61 and 62 are produced between the separator 60 and the positive electrode 70 or the negative electrode 80, a dendrite is likely to occur because of non-uniformity in the reactions in the gaps 61 and 62. The occurrence of the dendrite led to a problem of internal short-circuiting.
The present invention has been accomplished in order to solve the above problems, and intends to provide a nonaqueous secondary battery which can prevent a separator from wrinkling even when a polyvinylidene fluoride resin film with excellent electrolyte-retaining nature is used as a separator material and decrease an internal resistance to give improved cycle characteristic and excessive charging characteristic
In order to attain the above object, the nonaqueous secondary battery according to the invention uses a separator which comprises a polyvinylidene fluoride resin layer and a reinforcing material layer. Preferably the separator is a composite resin film with a polyvinylidene fluoride resin layer and a reinforcing material layer stacked thereon, or another composite resin film with a reinforcing material layer integrally filled with polyvinylidene fluoride resin. The composite resin film with a polyvinylidene fluoride resin layer and a reinforcing material layer stacked thereon can be a multi-layered composite resin film with a polyvinylidene fluoride resin layer and a reinforcing material layer stacked thereon, repeatedly. In this way, when the reinforcing material layer is stacked on the polyvinylidene fluoride (PVdF) resin layer is stacked, or the reinforcing material layer 11 is integrally filled with the polyvinylidene fluoride resin, even if the polyvinylidene fluoride resin swells because of retaining the electrolyte, its swelling is limited by the reinforcing material layer in the directions of width and length so that the separator does not wrinkle, thereby decreasing the internal resistance. Thus, a nonaqueous secondary battery with excellent cycle characteristic and excessive charging characteristic can be obtained.
In this case, the reinforcing material layer must be selected from the materials which do not swell even when they retain the electrolyte. Non-woven fabric of polyethylene terephthalate (PET), non-woven fabric of polypropylene (PP), non-woven fabric of polyethylene, microporous film of polyethylene, miciroporous film of polypropylene, etc. can be preferably used since they do not swell when they retain the electrolyte.
If the thickness of the reinforcing material layer is increased, the mechanical strength of the separator is also increased so that even if the polyvinylidene fluoride (PVdF) resin layer swells, the separator does not wrinkle. However, since the internal space of the battery is limited, if the thickness of the reinforcing material layer is increased, the thickness of the polyvinylidene fluoride (PVdF) resin layer must be decreased correspondingly. This leads to attenuation of its electrolyte-retaining nature. For this reason, it is preferred that the thickness of the reinforcing material layer is set at 30 xcexcm or less, more preferably so thin that even when the polyvinylidene fluoride (PVdF) resin film 21 swells, the separator 20 does not wrinkle.
Said polyvinylidene fluoride resin is preferably a vinylidene copolymer of vinylidene fluoride homopolymer or vinylidene fluoride and at least one selected from the group consisting of ethylene trifluoride chloride, ethylene tetrafluride, propylene hexafluoride and ethylene.
According to using a composite resin film with a polyvinylidene fluoride resin layer and a reinforcing material layer stacked thereon as a separator, composition in a surface direction is made more uniform.
According to using a composite resin film with a reinforcing material layer integrally filled with polyvinylidene fluoride resin as a separator, composition in a three dimension direction being uniform can be obtained.
According to using a multi-layered composite resin film with a polyvinylidene fluoride resin layer and a reinforcing material layer stacked thereon repeatedly as a separator, high efficiency per thickness can be obtained.